This invention relates generally to methods and apparatus for drying gases, especially natural gas, with a glycol; and more specifically relates to a method and apparatus of this type wherein the emissions of undesired organics which normally occurs during regeneration of the glycol are minimized.
Glycol dehydrators are commonly used to extract water from gas streams, including from produced natural gas streams to prevent problems associated with hydrate and ice formation and corrosion in the gas pipeline. Aromatic hydrocarbons are highly soluble in the triethylene glycol used in such glycol dehydrators and in the ethylene glycol used in large low temperature separation units. Therefore as the lean glycol solutions are contacted with the natural gas stream in an absorber, not only water, but various hydrocarbon species are absorbed. The water must be removed from the glycol to enable the latter's reuse in the drying of the gas. Accordingly, the water-containing (moist) glycol is conventionally heated in a so-called glycol regenerator, whereat the greater part of the water is vaporized, commonly in the form of steam and separated. A concentrated glycol having a low water content is recovered and recycled for further gas drying. This conventional type of operation is discussed in prior art patents such as U.S. Pat. Nos. 4,182,659 and 4,314,891, and is illustrated in simplified schematic fashion in the prior art system shown in FIG. 1. As seen therein the wet gas, as for example a natural gas 10 is provided to the bottom of an absorption column 12, emerges at the top of same and after passing through heat exchanger 13 is provided as a dry gas 14 for further use. During passage through the absorber 12, the gas contacts glycol 16, which is preferably triethylene glycol, in a countercurrent manner. The moist glycol emerges from the column at 18, passes through a throttle valve 20, is passed through a heat exchange loop 21 at the top of glycol regenerator 28, and provided to a flash tank 22 where it is expanded to the range of 2 to 10 atmospheres absolute. A two phase mixture forms at flash tank 22. Most of the mass will be present in the liquid phase as glycol and water. The bulk of the volume will be in the gas phase, mostly as light gases such as methane and propane. These light gases are undesired species that mainly have been absorbed in the glycol during drying. These light gases are vented at 24 as flash gas. Some of this flash gas may be vented at 23 as undesired emissions. The gas, 24, may also be used as fuel 25 to fire a reboiler 29 for regenerator 28. The glycol and small amount of water exits flash tank 22 at 27, then passes through a heat exchanger 26 where it is preheated and is then provided to the glycol regenerator 28.
The glycol regenerator 28 is heated, as for example from the gas fired reboiler 29 the flue gases from which exit at 30. The moist glycol is dried by the heat, and exits at line 31. It then passes through heat exchanger 26, is pumped back up to pressure by pump 33 and after passing through heat exchanger 13 is recycled back to absorber 12. Emissions from the heating are vented from the glycol regenerator 28 as regenerator off gas at 32. These emissions consist largely of the water which has been driven off from the glycol by the heating, which water is in the form of steam. However, there are also contained in the emissions a number of organic species, including benzene, toluene, ethyl benzene and xylenes (hereinafter collectively referred to as "BTEX"), and other volatile organic compounds ("VOCs") all of which (in the absence of control) are undesirably emitted to atmosphere.
The primary controls used to the present in order to eliminate or minimize organic emissions of the foregoing type are simple air and/or water-cooled condensers that produce a vent gas, an organic liquid, and liquid water containing organic compounds at a proportion of approximately 1400 parts per million (ppmw). This is comprised of about 350 ppm BTEX and 1050 ppm aliphatic hydrocarbons. Thus while BTEX control efficiency of these systems is high, a contaminated water is produced. This type of prior art control system is illustrated in the schematic showing of FIG. 2. Here the steam and organic emissions from glycol regenerator 28 pass via line 34 through a condenser 36 to the accumulator 38, where the emissions are separated into a water stream 40, an organic liquid 42, and vent gas 44. However, because of the nature of the emissions, the water stream 40 is highly contaminated as indicated. It may be noted that in some instance condenser 36 may include internally the functions of accumulator 38. Various thermal and catalytic incineration techniques have also been considered in the past as control techniques to eliminate the undesired organic species thus existing, but are costly to implement and not particularly effective or efficient.
In accordance with the disclosure of my aforementioned copending application, Ser. No. 825,540, emissions of volatile organic compounds which have been absorbed by the glycol during contact with the natural gas stream are controlled, while minimizing production of contaminants in the produced water stream. Pursuant to such disclosure, the vented steam and gaseous volatile organics (VOCs) from the glycol regenerator, are flowed as input to the bottom of a steam stripper column. The vapors exiting from the top of the steam stripper column are flowed to a cooling condenser to produce a condenser water stream having a relatively high content of the organic compounds. The condenser water stream is passed to the top of the steam stripper and then through the stripper in countercurrent relation to the gaseous VOCs and steam, in consequence of which the condenser water stream is stripped by the steam of substantial parts of its organics content, and emerges from the bottom of the stripper or column as a stripped water stream having relatively low content of the said organics. The stripped water stream may thereafter be passed to a water polishing stage whereat the organic content is further reduced to produce a polished water stream. Such further stage can take the form of an activated carbon adsorption system, an air stripper, or a polishing steam stripper. At least a portion of the polished water from these polishing systems can be utilized as coolant for the condenser where the latter includes a water-cooled stage or is generally of the water-cooled type. The vapors from the steam stripper are condensed into a condenser water stream, an organic vent gas, and an organic liquid. The vent gas may be used as a heating source for the glycol regenerator. In a further aspect of the prior invention, some of the energy content of the flash gas (see FIG. 1) may be used to compress the vent gas to render the latter more suitable for use as fuel at the regenerator and/or for flaring to atmosphere. Preferably the flash gas drives a jet compressor, whereby the combined flash and vent gases may be used for the aforementioned purposes.
While the aforementioned system is outstandingly effective for its intended purposes, it has been found in some instances that the steam stripper portion of the system introduces unnecessarily high costs and complexity. Accordingly, simplifications in such system have been sought which can yet achieve fully acceptable reductions in VOC emissions, and reduced contamination in the produced water streams.